Degassing technique of removing dissolved gas from liquid has been employed in various fields. As an example, removal of oxygen dissolved in water (H2O) is a generally employed way to prevent rust on a boiler and a pipe, and create water for precision cleaning. Of, if water is used as a solvent for a dye or a colorant used as ink in an ink-jet printer, and if gas in the form of a large number of tiny bubbles is dissolved in the water, a capillary tube of an ink-jet head is clogged. This leads to discharge failure. So, degassing technique of liquid has also become an important issue.
Meanwhile, technique of dissolving gas in liquid has also been in use. As an example, liquid prepared by dissolving carbon dioxide in ultrapure de-ionized water may be employed as a cleaning liquid used in a semiconductor manufacturing device. In this case, a desired liquid can be obtained by supplying carbon dioxide existing in the atmosphere to the ultrapure de-ionized water.
According to process conventionally employed for degassing of liquid or gas supply to the liquid, the liquid is heated or boiled, or is exposed in vacuum or in a low pressure condition. According to technique that becomes well known in recent years, a filter for dissolved gas control is added in a liquid feeding system, and pressure on the filter is controlled, thereby removing gas dissolved in liquid or adding the gas to the liquid.
In many cases, a membrane using a hollow fiber membrane is employed as the aforementioned dissolved gas control filter. The hollow fiber membrane employed as the membrane filter is formed by integrating fibers having hollows literally at the centers thereof, and having a diameter of from about 180 to about 240 μm and a thickness of from about 25 to about 50 μm. For degassing with the hollow fiber membrane, while liquid penetrates into the insides of the hollow fibers, the pressure of gas contacting the outer skins of the hollow fibers is reduced. In response, molecules of gas dissolved in the liquid move toward a direction under reduced pressure through gaps between the outer skins of the hollow fibers. Degassing is also realized in a converse way in the following structure: while the outer skins of the hollow fibers contact liquid, pressure inside the hollow fibers is reduced, so that the molecules of gas dissolved in the liquid move toward the insides of the hollow fibers.
Gas supply is realized in the way as follows opposite to that of degassing: by increasing the pressure of gas contacting the outer skins of the hollow fibers, molecules of the gas are caused to move into liquid having penetrated into the insides of the hollow fibers. Gas supply is also realized in a converse way as follows: by increasing pressure inside the hollow fibers, molecules of gas are caused to move into liquid contacting the outer skins of the hollow fibers.
A material of a structure having a large number of tiny holes of a size that allows passage of gas molecules but does not allow passage of liquid molecules is applicable as a material for the dissolved gas control filter. The dissolved gas control filter may be realized not only by a hollow fiber membrane but also by a stacked porous membrane, ceramic, or sintered metal powder.
Not only the dissolved gas control filter but also a generally employed filter suffers from clogging by the use of the filter, so that the filtering function thereof cannot be assured unless it is exchanged within a certain period of time.
If the dissolved gas control filter is an unused condition, it is capable of reducing the concentration of dissolved gas in liquid for example to 6.5 mg/L within a predetermined period of time. However, if the concentration of the dissolved gas after reduction within the predetermined period of time becomes 10.8 mg/L or more due to generation of clogging, it is determined that the dissolved gas control filter lost its filtering function. In this case, the dissolved gas control filter should be exchanged.
There are techniques, such as those disclosed in patent literatures 1 and 2, employed to check time of exchange of the aforementioned filter.